1. Field of the Invention
The present invention relates to a overcoat layer for a magnetic disk and more particularly to a very thin boron carbide overcoat layer which reduces spacing loss.
2. Description of the Related Art
A magnetic disk comprises a substrate, a magnetic layer on top of the substrate and an overcoat layer on top of the magnetic layer. The substrate provides support for the magnetic layer which can be magnetized with signal format and the overcoat layer protects the magnetic layer from wear due to contact with a slider. High density magnetic disk drives require that a magnetic head be positioned as close as possible to the magnetic layer of the disk. A typical drive has a density of about 1 gigabit per square inch (Gb/in.sup.2) of disk area. A goal, which has been set by the National Storage Institute Consortium (NSIC), is to develop a 10 Gb/in.sup.2 drive by the end of the 20th century. This means that bits must be written closer together in circular tracks on the disk, which is referred to as linear density, and the width of the circular tracks must be narrower, which is referred to as track density. The product of linear density and track density is referred to as areal density. As areal density is increased signal strength is decreased. As signal strength is decreased the magnetic head must be positioned closer to the magnetic layer of the disk in order to write and read weak signals with acceptable resolution. Two factors, which will be discussed hereinafter, impact on how close the magnetic head can be positioned to the magnetic layer of the disk, namely: (1) the flying height of the magnetic head above the disk and (2) the thickness of the overcoat layer on top of the magnetic layer of the disk.
The magnetic head is mounted on a slider which is supported by a suspension above a rotating disk. The suspension provides a slightly downward force which is counteracted by moving air which is generated by the rotating disk. The moving air provides an air bearing which supports the slider and magnetic head at a flying height slightly above the rotating disk. Considerable research is being undertaken to reduce the flying height in order to position the magnetic head as close as possible to the magnetic layer of the disk. The bulk of this research involves the configuration of the bottom surface of the slider which faces the disk, the bottom surface being referred to as an air bearing surface (ABS).
As stated hereinabove, the overcoat layer protects the magnetic layer from wear and damage caused by contact with the slider. When the disk starts rotating and stops rotating the slider takes off and lands on the disk. This is referred to as contact start and stop (CSS). In order to have acceptable durability the overcoat layer must withstand several hundred thousand contact starts and stops without noticeable wear. Unfortunately, the overcoat layer adds additional space between the magnetic head and the magnetic layer of the disk. The thickness of the overcoat layer adds to what is referred to as spacing loss. The typical overcoat layer is made from hydrogenated carbon. The thickness of this layer is 150 .ANG. or more in order to be acceptably durable. There is a strong felt need to reduce this thickness in order to position the magnetic head closer to the magnetic layer of the disk.
Boron carbide has been recognized for its superb low coefficient of friction and wear capabilities. See "Dry Friction and Wear of Chemically Vapour Deposited Boron Carbide Coatings" by Rey, Kapsa and Male in "Surface and Coatings Technology," 36 (1988) 375 and "Tribochemical interactions of boron carbides against steel" by Gogotsi, Koval'chenko and Kossko in "Wear", 154 (1992) 122-140. The first article investigated 30 micron thick boron carbide layers and the second article reported on a 10 micron thick boron carbide layer for automobile engine components. U.S. Pat. No. 5,275,850 suggests chemical vapor deposition (CVD) of 200-300 .ANG. of carbon and selected elements in specific ratios to form a protective layer for a magnetic disk. The patent also refers to various adhesion layers between the magnetic layer and the protective layer of the disk. U.S. Pat. No. 4,840,844 refers to 50-2000 .ANG. thick boron carbide protective layers for (1) magnetic disks where the carbon constituent increases from the underlaying magnetic layer toward the surface of the disk or (2) where the boron carbide layers are laminated with carbon layers. The patent did not provide any examples of protective layers less than 200 .ANG..